Seasonal influence on equine follicle dynamics

Recent innovations in ultrasonographic techniques have advanced our knowledge on the effects of seasonality on the changing populations of antral follicles in mares. The seasonal patterns in the monthly incidence of ovulations is at least partly a function of daylength, even at latitudes as low as 10o where the longest day may be less than 13 hours. Follicle dynamics involves a mid-anovulatory phase in winter when the follicles may not exceed 16 or 17 mm. A minor follicular wave (no dominant follicle) ends the midanovulatory phase and begins a resurging phase, raising the diameter of the largest follicle to at least 21 mm. The largest follicle of subsequent minor waves reaches a similar diameter forming an approximate plateau. However, in some mares, the later portion of the resurging phase or the entire phase may consist of a period of major anovulatory waves in which the largest follicle approaches or is similar to the diameter of a dominant ovulatory follicle. In one study, the length of the resurging phase was a mean of 52 days and consisted of minor waves, unless a period of major anovulatory waves occurred. Each minor or major wave is stimulated by an FSH surge, and the surges are similar regardless of the diameter reached by the largest follicle. Results of a Doppler study has led to the hypothesis that the future dominant-sized (≥ 30 mm) follicles of the period of major anovulatory waves develop during an LH deficient environment beginning when the follicles are as small as 15 mm. These future dominantsized anovulatory follicles are already dysfunctional at about 20 mm as indicated by an increase in circulating FSH. Despite an aberrant vasculature and low concentrations of estradiol, free IGF-1, inhibin-A, and VEGF, the follicles continue to expand at a rate comparable to the growth of an ovulatory follicle. The effects of daylength on follicle dynamics continue through the ovulatory season. For example, follicular activity of the estrous cycle is greater during the first half of the season as indicated by more follicles ≥ 20 mm, greater diameter of largest follicle, and greater incidence of minor and major anovulatory waves. After the last ovulation of the year, most mares, during a receding phase, have an increase in follicle activity at the time ovulation would have occurred. A major anovulatory wave or a minor wave develops during the increased follicle activity. Concentrations of LH are greater in association with major waves than with minor waves, but the wave-stimulating FSH surges are similar. A persistent corpus luteum forms from the site of the last ovulation of the year in about 25% of mares. In these mares, a period of major anovulatory waves may ensue until the corpus luteum regresses.